The present invention relates to a method for forming a film using an organometallic compound or complex as a source material and to a method for fabricating a semiconductor device in accordance with the film forming method.
Recently, as the performance of electronic units has been enhanced, it has become increasingly necessary to use a large-capacity memory such as a dynamic random access memory (DRAM) or a ferroelectric random access memory (FeRAM) or a high-performance device such as an MFS or MFIS transistor as an capacitor thereof. In all of these devices, a capacitive insulating film thereof is made of a material with a high dielectric constant (hereinafter, referred to as a "high-dielectric-constant material") or a ferroelectric material. In FeRAMs and MFS or MFIS transistors, an insulating film is made of a ferroelectric material such as PZT. To realize a DRAM with a storage capacity of 1 gigabits or more, its capacitor for storing electrical charge thereon should also be made of a high-dielectric-constant or ferroelectric material such as Ba--Sr--Ti--O (BST) and Pb--Zr--Ti--O (PZT).
On the other hand, since a DRAM is required to include as large a number of devices as possible on a single chip, the individual devices, including capacitors, should be downsized. For that purpose, the size of a capacitor has been reduced in various manners, while maintaining a sufficient storage capacity by increasing the surface area of the capacitive insulating film. For example, a cylindrical capacitor may be formed or the surface of a storage node may be roughened to form a large number of tiny stepped portions thereon. A similar downsizing requirement is also imposed on FeRAMs and MFS or MFIS transistors.
Thus, in forming a high-dielectric-constant or ferroelectric film, a CVD process is used most often, because a film can be formed over very small stepped portions with good step coverage according to the CVD process. This is why research and development has been vigorously carried on to form a film with good ferroelectric properties by a CVD process.
Examples of CVD processes applicable to deposition of a BST or PZT film include a thermal CVD process. According to the thermal CVD process, organometallic complexes, containing the constituent metal elements of the BST or PZT film, are used as source materials. These organometallic complexes are dissolved in a polar solvent such as butyl acetate or tetrahydrofuran (THF), vaporized, and then introduced into a reaction chamber, thereby causing a chemical reaction such as decomposition or combination among them on a heated substrate. In this case, a plasma-enhanced CVD process, in which the reaction on the substrate is accelerated by plasma generated within the reaction chamber, is sometimes adopted. Also, in the thermal or plasma-enhanced CVD process, a plurality of source materials may be mixed at a predetermined ratio by various techniques. For example, according to a technique, respective solutions of organometallic complexes are mixed at the predetermined ratio and then vaporized. Another technique is vaporizing respective solutions of organometallic complexes and solvents and then mixing the resultant gases at the predetermined ratio.
For instance, in depositing a BST film by a CVD process, three organometallic complexes of Ba(DPM).sub.2, Sr(DPM).sub.2 and Ti(O-iPr).sub.2 (DPM).sub.2 (where DPM is dipivaloylmethanato) are used as respective source materials, dissolved in butyl acetate at room temperature and then mixed at a predetermined weight ratio. Next, the mixture is introduced into, and vaporized by, a vaporizer that has been heated up to about 220.degree. C. Thereafter, these three organometallic complexes vaporized are introduced into a reaction chamber, in which a substrate has been heated up to about 400.degree. C. to about 700.degree. C. And then these three organometallic complexes vaporized are allowed to react with each other on the substrate, thereby forming a BST film thereon.
FIG. 6 illustrates molecular states when respective organometallic complexes are allowed to react with each other in a reaction chamber. As shown in FIG. 6, organometallic complexes are more likely to combine with each other to form a copolymer rather than remaining homopolymers, generally speaking. Accordingly, a variation in vaporization temperature or decomposition happens easily. Thus, in many cases, the formation of such a copolymer is prevented by a steric hindrance state, which has been created by a copolymer like a dimer or trimer through the coordination of a so-called "adduct" such as a tetraglyme group.
It is known, however, that a large number of carbon compounds are left within a film formed by a CVD process using these organometallic complexes as source materials. These residual carbon compounds have not caused a serious problem yet in memories like DRAMs and FeRAMs. Nevertheless, the existence of carbon compounds in a dielectric film affects the electrical properties such as dielectric ones. Also, if the residual carbon compounds change into mobile ions, then the ions move along with the electric field resulting from the operation of capacitors and are segregated, thus possibly deteriorating the reliability of the device. Moreover, as the memory cells of a DRAM or FeRAM are further downsized, various inconveniences might be created by the existence of such carbon compounds. MFS and MFIS transistors, on the other hand, are particularly likely to operate erroneously due to the mobile ions. Thus, there is much concern about deterioration in reliability of transistors such as these.